The present invention relates to settling of thick suspensions.
Suspensions are formed in many types of technology, usually as industrial wastes. Among suspensions are tailings, pulps, refuse, and contaminated earth. Tailings are the most important Types  types of wastes from both economical and ecological standpoints.
For better extraction of ores the rocks are ground to silt-and clay-size fraction. After ore benefaction the tailings are transported to tailings ponds for storage. The solid concentration in the initial slurry is different. The slurry discharged from the mill contains usually from 15 to 55% of solids.
The suspension in tailings ponds settles with decelerating rate. The mass is separated into supernatant and rather dense slurry containing higher concentration of solids.
Following the initial period of settling, the process becomes extremely slow. An idea of the duration of the process may be gained from the distribution of unit weights of sludge in tailings ponds. The following data refer to a disposal of tar sand tailings. The accumulation of tailings began in 1966; the measurements were made in 1977.
Relationship between d and xcex3m is linear. That testifies to continuing of settling, because a definite value of unit weight in the whole mass corresponds to the critical state. Condensation of tailings after 11 years of settling is far of the end. The tailings of the pond are in the subcritical state (see below).
The time of total settling of solids to a compact deposit that can support load is unknown. It is supposed that the time of total settling is varying anywhere between 100 and 1000 years. This time period is too long to be checked.
Mine tailings represent the largest tonnage of industrial waste produced. Billions of tons off wastes are produced annually. Unused tailings are stored in tailings ponds. The area and volume of tailings ponds increase everywhere. Tailings ponds occupy the floor of valleys. Valuable material goes to waste. Tailings ponds and contaminated earth represent a serious and major environmental problem. The visual effect of the landscape of tailings disposal is despondent. However the major ecological effect is the surface and ground water pollution. The contaminated water contains heavy metals, mill reagents, sulfur compounds, etc. Uranium tailings are a serious source of radioactive contamination.
Tailings ponds collect all precipitation falling in basins upstream of tailings dams. The fresh rain and melted snow water is mixed with the polluted technological water of tailings and gets contaminated too. Its ecological effect cannot be overestimated.
Tailings ponds may cause major disasters on bursting of tailings dams. The bursting of tailings dams is not infrequent. During last decades such failures occurred at El Cobre Dam, Chile in 1965, coal waste pile in Aberfan, Wales, England in 1966. Mulfalire mine in Africa in 1970, coal refuses at Buffalo Creek, West Virginia in 1972, Mochikoshi Dam, Japan in 1978, etc.
Twenty years ago Fitch wrote: xe2x80x9cThere exist no models, theories, or design procedures, which could be accepted as well proven at the present time. We hope that we are making progress, but those applying existing theories or procedures for practical design should accept none as gospel.xe2x80x9d Very little has been changed during time elapsed.
It is the object of this invention to provide an effective and inexpensive method for settling of suspensions.
Suspensions containing liquid and solid constituents settle only partly. They form a viscous or jelly-like mass that does not settle completely itself with time. They can be dehydrated artificially only by use of chemicals or by drying. The reason why suspensions do not settle remained unidentified up to present.
The invention provides an unobvious method of processing that satisfies the long-existing requirements of industry. The industry incurs immense material losses measured annually by billions of dollars. The environment suffers from great impact. In spite of these facts the problem remains unsolved for more than a century.
The situation was so hopeless that the problem was considered officially as an unsolvable one. According to the definition, a suspension is a xe2x80x9cliquid carrying throughout its volume in extremely fine subdivision an insoluble substance (solid or another liquid) which will not settle under gravity nor can be filtered without special treatment such as addition of chemical agentsxe2x80x9d (U.S. Patent and Trademark Office, U.S. Patent Classification Definitions. Class 210xe2x80x94Liquid Purification or Separation. June 1955, page 210-5).
Such behavior of suspensions is a result of action of the suspension force. This force is generated by friction between immovable water and sinking solid particles. This force had been discovered, and its properties were investigated by the inventor. That made it possible to propose the present method of settling without any addition of chemicals, or other special treatment.
An effective method of settling of tailings and the like is of great importance. It will permit the recovery and use of the mineral resources contained in waste, and accumulated in tailings ponds. An incredible quantity of these mineral resources is lost: one third of phosphate mined in Florida, one half of tin mined in Bolivia and one fifth of the world""s tungsten. Settling of tailings has a number of advantages. The tailings after settling can be used as building and road materials. Valuable land occupied by tailings ponds can be returned to the national economy. Water and soluble chemical reagents used in technological process will be recycled. The demand for fresh water will be kept to a minimum. Finally the threat of disasters on bursting of tailings dams can be removed.
Settling of tailings can be made directly in concentration mills. Major expenses for transportation of tailings, their thickening, construction of tailings dams, management of tailings ponds, protection of groundwater from contamination will be saved. Tailings from a major debit item turn into a major credit item.
The method can be used also for purification of liquids from impurities.